This relates to integrated circuits with power supply circuitry. Integrated circuits often contain volatile memory elements. Typical volatile memory elements are based on cross-coupled inverters (latches). Volatile memory elements retain data as long as the integrated circuit is powered. For example, integrated circuits may include volatile memory elements such as static random access memory (SRAM) cells.
Integrated circuits such as programmable integrated circuits may include volatile memory elements such as configuration random access memory (CRAM) cells that are loaded with configuration data. Each CRAM cell has an output that provides a control signal that is used to enable or disable a corresponding pass transistor in logic circuitry. Pass transistors (pass gates) are typically formed from n-channel transistors.
Pass transistors are turned on or off to selectively pass or block passage of logic signals in response to the control signals provided by the CRAM cells. In an effort to increase performance, pass transistors can be turned on using elevated gate control signals (i.e., by overdriving the gates of the pass transistors at an elevated voltage level greater than the nominal positive power supply voltage that is supplied to the remainder of the logic circuitry). To provide such overdriving capabilities, the power supply circuitry may supply the CRAM cells with an elevated positive power supply voltage.
Pass transistors overdriven in this way may, however, suffer from potential transistor breakdown and reliability issues. Metal-oxide-semiconductor field-effect transistors (MOSFETs) typically exhibit a gate breakdown voltage that varies with temperature. For example, a pass transistor may exhibit a breakdown voltage level of 1.1 V when the integrated circuit is operating at 20° C. and may exhibit a breakdown voltage level of 1.0 V when the integrated circuit is operating at 50° C. (i.e., the gate breakdown voltage level reduces as temperature increases).
Conventional ways of overdriving pass transistors involve biasing the gate of pass transistors to the same elevated positive power supply voltage level across all operating temperatures. If an integrated circuit is operating at temperatures below a given threshold, the pass transistors may function satisfactorily. If, however, the integrated circuit is operating at temperatures greater than the given threshold, the pass transistors may suffer from degraded breakdown voltage and may be overstressed, resulting in dielectric breakdown that renders the pass transistors inoperable.